The client by default will prompt you to unlock your wallet so it can denominate and mix, the default is 2 rounds of mixing. There will be a small fee of 0.01DRK per round of mixing. In the settings you can select more rounds of mixing and you can also indicate the soft limit of how many anonymous coins you want, the client will then denominate and mix a quantity that is relatively close to your setting ( again it wont be exact.), if you select additional rounds of mixing it will take more time to denominate, make sure your wallet is open, and your internet connection active while you go through the process. If you dont do anything and stay with the default settings all you have to do is wait.